The invention relates generally to thyristors, and more particularly to thyristors of the type having a localized region within the semiconductor body at which voltage breakover initiates when the forward anode-cathode voltage exceeds the forward breakover voltage.
Switching a thyristor into conduction by exceeding the forward breakover voltage can result in damage to the device. One cause of damage is the small size of the voltage breakover region which is initially turned on. If the current rises rapidly to large values before a sufficient portion of the main current carrying part of the device turns on, significant power dissipation occurs. As a result of local overheating in the initial breakover region, the device fails.
Techniques have been developed for localizing the initial breakover region within a thyristor. One method is to produce a localized region of lower resistivity in the N-substrate. A region of lower resistivity can be produced by passing a beam of neutrons through the semiconductor material. By the well known process of neutron transmutation, the silicon is partially converted to phosphorus, which raises the level of N-type doping within the irradiated area. The result is a localized reduction in resistivity, which creates a localized initial breakover region at the adjacent pn junction. The position of the breakover region can be advantageously located within the thyristor body. For example, it is known that positioning the voltage breakover region beneath a centrally located gate electrode is known to produce more rapid thyristor turn-on.
Localizing and positioning the voltage breakover region in a thyristor does not solve the problem of large power dissipation during initial breakover, however. The large currents which occur at breakover must still initially pass through a small junction area. Unless external circuit conditions control the initial current, overheating and destruction of the breakover junction will lead to premature failure of the device.